A ludicrous, borderline-nonsensical supernatural concoction with a slightly redeeming sense of its own silliness.

Set in a world of scantily clad demon hunters, bisexual warlocks, and a host of vampires and werewolves apparently on loan from “The Twilight Saga,” “The Mortal Instruments: City of Bones” is a ludicrous, borderline-nonsensical supernatural concoction with a slightly redeeming sense of its own silliness. Even by the genre’s lax standards of plausibility, not a whole lot coheres in this first adaptation of Cassandra Clare’s bestselling urban-fantasy series, a muddled mash-up of Stephenie Meyer, J.K. Rowling and Joss Whedon with a little “Men in Black” thrown in for good measure. While its tattooed torsos, fantastical f/x and precociously kinky undertones suggest a focus group’s notion of what teenagers want from their entertainment, this Canadian-German co-production could have a hard time overcoming the B.O. curse of recent fantasy fare like “Beautiful Creatures” and “Percy Jackson: Sea of Monsters.” Still, the Screen Gems release should benefit from a five-day opening weekend devoid of much direct competition.

The next film in the series, “The Mortal Instruments: City of Ashes,” is already set to begin production this fall with director Harald Zwart (“The Karate Kid,” “The Pink Panther 2,” “Agent Cody Banks”) back at the helm, suggesting a measure of studio confidence that will be adjusted upward or downward depending on how “City of Bones” performs. Still, at a bloated 130 minutes, this overstuffed f/x extravaganza has that desperate, cram-it-all-in quality typical of a franchise-starter wannabe, suggesting the work of individuals not entirely convinced they’ll be back for the sequel, or that there will even be a sequel.

Jessica Postigo Paquette’s screenplay all but crumbles beneath the weight of its convoluted mythology and hormonal melodrama, as well as its narrative obligations as an origin story of sorts. The film spends an inordinate length of time waiting for dark-haired Clary (Lily Collins) to realize that, although she likes to argue with her mother (Lena Headey) and attend crappy poetry readings with her nerdy best friend (Robert Sheehan), she is not, strictly speaking, just an average New York teenager. In fact, she’s descended from a long line of Shadowhunters, a race of magically gifted leather-clad fighters who do battle with the demons that walk undetected among mankind.

One of the reasons Clary belongs in the Shadowhunters’ company is that she can see them, a gift not granted to ordinary muggles (er, “mundanes,” as they’re called here). And what a sight they are: Clary’s guide to this invisible world of rites and ruins is Jace (Jamie Campbell Bower), a mildly androgynous blond heartthrob whose presence, more than that of any witch or vampire, is what primarily defines this story as a fantasy. Jace is lusted after not only by Clary, but also by his Shadowhunter buddy, Alec (Kevin Zegers), who nonetheless keeps his desires under wraps. Far less inhibited is Magnus Bane (Taiwanese model-actor Godfrey Gao), drolly described as “the high warlock of Brooklyn”; his first appearance in heavy guyliner, piercings and a disturbing blazer-and-boxers combo is a thing of very dark magic indeed.

That scene sets up the film’s subtlest joke, suggesting it can be hard to tell the difference between a supernatural shindig and certain enclaves of New York nightlife. (The costumes were designed by Gersha Phillips.) These outre touches and flashes of self-aware humor are what sustain passing interest in Zwart’s film, providing welcome distractions from its needlessly complicated, none-too-interesting story, in which Clary tries to track down her missing mother and retrieve something called the Mortal Cup — a much-coveted artifact whose specific properties remain an utter mystery even after you’ve heard everyone whisper about its unfathomable importance for the better part of two hours.

The plot is essentially one long tease, anyway; it’s established early on that Clary’s knowledge of her Shadowhunter past has been deliberately suppressed, a fairly convenient device from a screenwriting standpoint (it’s not a plot hole, it’s a memory block!). In short, “The Mortal Instruments: City of Bones” is the sort of frenetic fantasy that piles on the pyrotechnics, the prosthetics and the simpering romantic interludes in lieu of any genuine sense of wonder or emotion. Those who fall outside the film’s intended viewership may pass the time by contemplating the surprising sexual conservatism of the Shadowhunter universe, or perhaps the curious resemblance between Jace and a character from a far superior epic franchise (Cassandra Clare achieved her early fame as a popular and controversial figure in Harry Potter fan-fiction circles).

Even when the story requires her to dress up like a fetish-ball Cinderella, Collins can’t do much to engage interest in a character who is by turns stubborn, angsty, indecisive and a bit slow on the uptake; having previously played Snow White in “Mirror, Mirror,” the actress may want to reconsider taking on too many films of this ilk in which she is not allowed to be the most interesting thing onscreen. As one of Clary’s two love interests, Bower sells his brooding-Brit act persuasively enough and wields a sword handily in the generally incoherent action sequences. On the adult side of the equation, Headey is a fiercely watchable presence in her too-brief moments; CCH Pounder and “Mad Men’s” Jared Harris enliven the proceedings as vaguely sinister characters operating on the story’s periphery; and Jonathan Rhys Meyers momentarily jolts the film awake as a louche Shadowhunter who expands the film’s stew of pop-cultural influences to include “Star Wars.”

Much of the picture’s relatively modest budget seems to have been applied toward scenes of metamorphosis, in which hitherto benign characters suddenly shift into demon mode, while some of the more bizarre gadgets on display — a fire-generating pentagram, a glowing aquatic memory portal — are well visualized by the f/x team even when the script is at a loss to explain their purpose. As ever, Toronto makes a workmanlike stand-in for New York, while production designer Francois Seguin brings rote but effective Gothic touches to bear on his sets for the Institute (the Shadowhunters’ Hogwarts-style safe house) and the underground catacombs that explain the second half of the film’s ungainly title. Atli Orvarsson’s unmemorable score is supplemented with snippets of Bach’s Goldberg Variations, which tie into one of the script’s better gags.

Incidentally, in a departure from usual practice, the Los Angeles press screening of “City of Bones” was preceded by a trailer for Sony’s upcoming “One Direction: This Is Us,” while the screening of “One Direction: This Is Us” was preceded by a trailer for “City of Bones” — all presumably to maximize awareness among two target audiences with a fair amount of overlap.